Vapour phase esterification of free fatty acids

ABSTRACT

A method is presented for producing biodiesel from a triglyceride feedstock. The feedstock is pretreated by thermal cracking or rapid pyrolysis to form a middle distillate fraction rich in fatty acids. The middle distillate fraction is then treated by vapour phase esterification under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a biodiesel stream. The biodiesel stream can be treated with a basic solution to convert residual free fatty acids to non-foaming metallic soaps, which are separated by known means. A method is also provided for producing a biodiesel/naphtha mixture, in which a triglyceride feedstock is pretreated by thermal cracking or rapid pyrolysis to produce a middle distillate fraction, a naphtha stream and a gas stream. The naphtha stream and the middle distillate fraction are then treated by vapour phase esterification under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a mixed biodiesel/naphtha stream.

FIELD OF THE INVENTION

The present invention relates to a method of producing biodiesel from triglycerides combining thermal cracking and esterification.

BACKGROUND OF THE INVENTION

In recent years, the area of biodiesels has drawn a great deal of attention. Biodiesels are fuels produced from the esterification of biomass-derived oils with alcohol. Biodiesel can be produced from triglyceride sources such as canola, corn, soybean, palm etc.

Another potential source for biodiesels are the waste triglycerides from animal rendering facilities and waste cooking oils, such as those found as restaurant trap greases. However, this potential is presently still under-explored and waste triglycerides, trap grease in particular, are most commonly dumped into landfills. Waste triglycerides often have high contaminants content, including bacteria, which must effectively be removed before processing. Furthermore, waste triglycerides tend to have a high content of free fatty acid (FFA), anywhere in the range of from 50% to 100%. Mixtures of free fatty acids and triglycerides have been found to be very difficult to convert to useful fuels by any traditional methods.

Traditional methods of producing biodiesels include transesterification and esterification with alcohol using either an acid or base catalyst. However, the high FFA content in waste triglycerides causes undesirable soap formation in base catalyzed esterification processes, rendering this process inoperable.

Waste triglycerides are also often heavily contaminated by, for example, bacteria, detergents, silts and pesticides. These contaminants must be removed before esterification can take place, without adding significant additional cost to the overall processes.

One known method of processing high FFA feedstocks involves adding glycerol to the feedstock to convert FFA's to mono- and diglycerides, followed by conventional alkali-catalyzed esterification. This method addresses the issue of high FFA content but does not treat or remove contaminants. A second method involves pre-treating an FFA-rich triglyceride feedstock with an acid catalyst to convert FFA to alkyl-esters and reduce FFA concentrations to less than about 0.5%, followed by traditional base-catalyzed esterification. This method again, only deals with the issue of high FFA content and not high contaminant levels.

An alternate method involves performing both esterification and transesterification of triglycerides using a strong acid such as H₂SO₄ or sulphamic acid; however the product clean up is cumbersome and usually involves neutralization of the acidic catalyst and the removal of resulting salts. Acid ion-exchange resins are another option, but due to possible resin degradation esterification must be carried out below the resin degradation temperature, which significantly slows down the process. As well, water formation by FFA esterification prevents this process from going to completion.

Reaction time required for typical bio-diesel production by acid esterification ranges from 10 hours to 20 hours, which makes acid esterification of fatty acids an industrially unattractive route for fuel production. When esterification temperature is raised above 100° C., satisfactory conversion can be achieved within 8 to 12 hours. However, carrying out acid esterification at above water's boiling point diminishes alcohol solubility in the methyl-esters. This is undesirable since, in order to bring esterification to near completion a high alcohol concentration must be maintained in the reactant mixture and this becomes difficult due to limited alcohol solubility.

Thermal cracking of clean triglycerides under typical cracking conditions with and without catalyst has been attempted, but this process was found to yield mainly naphtha, not diesel fuels. Furthermore, in typical thermal cracking of clean or waste triglycerides in the presence of a catalyst, there is a tendency for coke formation to occur on the catalyst, resulting in rapid deactivation.

It is therefore greatly desirable to find a method of converting triglycerides, and in particular low quality and waste triglyceride feedstocks, to biodiesel that is both efficient and economical. It is also desirable to find ways of dealing with contaminants and high FFA content in waste triglyceride feedstocks so that they can be converted into usable fuels.

SUMMARY OF THE INVENTION

The present invention thus provides a method of producing biodiesel from a triglyceride feedstock, comprising pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides, to form a middle distillate fraction rich in free fatty acids. The middle distillate fraction can then be treated by vapour phase esterification under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a mixed biodiesel/diesel stream. The mixed biodiesel/diesel stream can then be treated with a basic solution to convert residual free fatty acids to non-foaming metallic soaps, which non-foaming metallic soaps can be separated by vacuum distillation, centrifugation, filtering or combinations thereof.

The present invention also provides a method of producing a biodiesel/naphtha mixture from a triglyceride feedstock. The method involves first pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides, to produce a middle distillate fraction rich in free fatty acids, a naphtha stream and a gas stream. Next, the naphtha stream and middle distillate fraction are treated by vapour phase esterification under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a mixed biodiesel/naphtha stream. The mixed biodiesel/naphtha stream can then be treated with a basic solution to convert residual free fatty acids to non-foaming metallic soaps, which non-foaming metallic soaps are separated by vacuum distillation, centrifugation, filtering or combinations thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in further detail with reference to the following drawings, in which:

FIG. 1 is a flow sheet of a first preferred process for carrying out the present invention; and

FIG. 2 is a flow sheet of a second preferred process for carrying out the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present process employs a novel combination of thermal cracking followed by solid acid esterification under vacuum and elevated temperatures to convert triglycerides, and particularly low quality and waste triglycerides, into usable biodiesel. In the present process, thermal cracking is used as a pre-treatment step to break down the triglycerides into a broad range of free fatty acids and lower molecular weight components. Thermal cracking also serves to remove contaminants found in waste triglycerides, which can cause problems downstream. The resulting product from the cracking step can then be treated by solid acid esterification under vacuum and elevated temperatures to convert fatty acids into alkyl esters (biodiesel). The esterification is carried out in the vapour phase.

For the purposes of the present invention, thermal cracking is considered to loosely cover the process of breaking down large molecules into smaller molecules at a predetermined temperature and pressure. Rapid pyrolysis of triglycerides can also be used in the present process and is considered to be encompassed by the term thermal cracking. Details of rapid pyrolysis are given below.

A flow diagram of the process steps and streams of one embodiment of the present invention is shown in FIG. 1. A feedstock 12 of triglycerides, and particularly low quality or waste triglycerides, is fed to a thermal cracking unit 10. The feedstock 12 can be any variety of triglyceride including oils such as canola, soy, corn, palm, cottonseed, mustard seed, fish or algae oils and waste or low quality triglycerides such as restaurant trap greases, waste greases from animal rendering facilities and other forms of waste oils and greases and low-quality vegetable oils. The feedstock stream 12 can be heterogeneous in nature and can contain water and other contaminants. The triglyceride feedstock stream 12 can also have free fatty acid (FFA) content as high as 50 to 100 wt. %. In an optional embodiment (not shown), the triglyceride feedstock 12 may be filtered to remove any macroscopic contaminant particles prior to thermal cracking.

In the thermal cracking unit 10, triglycerides in the feedstock stream 12 are destroyed since they are converted into free fatty acids, thus forming a mixture of free fatty acids and conventional hydrocarbons, such as paraffins, olefins and aromatics. Thermal cracking is preferably carried out at mild cracking conditions which, for the purposes of the present invention, are described as an operating temperature preferably in the range of from 390 to 460° C., more preferably from 410 to 430° C., and preferably at an operating pressure of from 0 to 60 psig (6.9 to 515 kPa), more preferably from 30 to 40 psig (308 to 377 kPa). Thermal cracking produces various fractions including gases 14, naphtha 16, middle distillate 22, and residue 18. Gases mainly comprise of CO, CO₂, hydrogen, methane, ethane, ethylene, propane, and propylene. Contaminants from the feedstock 12 end up in the residue stream 18.

It was noted that the mild thermal cracking conditions used in the present invention to crack the feedstock 12 produces a mainly diesel-like fraction, having a boiling range of between 165° C. and 345° C., rather than naphtha (IBP to 165° C.), as was produced from thermal cracking of triglycerides at higher temperatures and pressures.

The middle distillate fraction 22 makes up more than half of the thermally cracked product and has been found to have suitable characteristics for further treatment by esterification. The middle distillate fraction 22 is rich in C16 and C18 fatty acids, comprising free fatty acids formed from thermal cracking of triglycerides, the original free fatty acids present in the feedstock and conventional hydrocarbons. Middle distillates typically encompass a range of petroleum equivalent fractions from kerosene to lubricating oil and include light fuel oils and diesel fuel. In one embodiment of the present invention the middle distillate fraction 22 was found to have a boiling point range of from 150 to 360° C., and more preferably from 165 to 345° C. The middle distillate fraction 22 still has some fuel quality issues such as high viscosity, high acid number, high cloud point and high concentrations of nitrogen and/or sulphur.

The present invention incorporates a vapour phase esterification process that overcomes the alcohol solubility issue and thus substantially accelerates acid esterification rates. In the present invention, acid esterification is operated under vacuum and high temperature, higher than normally used in liquid esterification, to achieve free fatty acids conversion higher than 97% in less than 10 minutes. High temperature accelerates the reaction rate and high vacuum and temperature ensure all components are in vapour phase. By operating in the vapour phase, it is possible to increase alcohol concentration by simply increasing the feed rate of the alcohol, since the alcohol solubility is no longer an issue.

The middle distillate fraction 22 is fed to an esterification unit 20, where vapour phase esterification is carried out in the presence of an alcohol stream 24 and a solid acid catalyst to produce alkyl esters (biodiesel). The esterification process is carried out at a temperature preferably ranging from 150 to 350° C., more preferably from 200 to 250° C. The esterification process operates under a vacuum, preferably in the range of 0.1 to 1.16 psia (6 to 60 mmHg), and is more preferably 0.1 to 0.58 psia (6 to 30 mmHg). The alcohol stream 24 can be any suitable alcohol known in the art, or mixtures thereof. The alcohol stream 24 is preferably methanol. The ratio of middle distillate stream 22 to alcohol 24 is preferably in the range of from 3:1 to 0.1:1 and is more preferably in the range of from 2:1 to 1:1.

Residence time in the esterification unit 20 can range from 6 to 425 minutes and preferably ranges from 6 to 43 minutes. For the present purposes, residence time is defined by dividing the catalyst volume by the total liquid feed rate.

The ability to conduct the esterification at higher temperatures is further advantageous since this circumvents the catalysis quenching by water. Since water is a co-product of acid esterification, it can detrimentally quench the esterification reaction if not removed continuously. In the present invention, as water forms by esterification, it immediately evaporates from the catalyst surface, thereby avoiding deactivation of the esterification catalyst.

The solid acid catalyst is preferably chosen from super acids such as, for example TiO₂ solid support doped with Zr(SO₄)₂, SnO₂ doped with sulphuric acid, and sulphated zirconium oxide (ZrO₂/SO₄). Other solid acids suitable for the current application are superacids including sulphated iron oxide or halogenated alumina, sulphated tin oxide, trifluoromethyl-imines (R₁CF₃CNR₂, where R₁ and R₂ are hydrocarbon chains), tungstated zirconia-alumina (W/SiZr—Al), silica-supported aluminum chloride.

Free fatty acids can be acid esterified by the following reaction, here shown with the alcohol optionally being methanol:

The water byproduct can inhibit the reaction, and may prevent esterification from proceeding to completion. As mentioned above, esterification at high temperatures and under vacuum conditions has been surprisingly found to alleviate this problem in the present invention.

The present inventors have conducted acid esterification of middle distillate derived from thermal cracking of triglycerides using the methods of the present invention. Results are given in Table 1 below.

TABLE 1 Acid esterification of middle distillates derived from thermal cracking of triglycerides Residence Feed to FFA Temp Pressure time** methanol conversion* Product Run ID Catalyst*** (° C.) (mmHG) (min) ratio (by wt.) % appearance Feed: Thermally cracked trap grease 060714 SnO₂/SO₄ ² ⁻ 300 77 425 1:1 70.5 (73.5) Brown 060621 SnO₂/SO₄ ² ⁻ 252 197 425 2.8:1   20.7 (16.3) Brown, Partially Crystallized 060711C SnO₂/SO₄ ² ⁻ 250 760 425 1:1 78.3 Black 060829 ZrO₂/SO₄ ² ⁻ 250 57 425 1:1 84.8 Yellow/Brown 060811 ZrO₂/SO₄ ² ⁻ 200 57 425 1:1 99.3 (99.7) Very light brown 060822 TiO₂/Zr(SO₄)₂ 200 57 425 1:1 82.8 Light yellow Feed: Thermally cracked palm oil 060914 ZrO₂/SO₄ ² ⁻ 200 5 42.5 1:1 91   Light yellow 060928B ZrO₂/SO₄ ² ⁻ 250 5 42.5 1:1 98.5 Light yellow 060929A ZrO₂/SO₄ ² ⁻ 250 5 8.5 1:1 97.3 Light yellow 061107 ZrO₂/SO₄ ² ⁻ 250 5 5.7 1:1 95.0 Light yellow 061103B ZrO₂/SO₄ ² ⁻ 250 5 8.5 1:1 98.0 Light yellow 061106A ZrO₂/SO₄ ² ⁻ 250 10 8.5 1:1 97.7 Light yellow 061106B ZrO₂/SO₄ ² ⁻ 250 25 8.5 1:1 97.1 Light yellow *FFA conversions were obtained by the change in TAN (total acid number) of the feed and the product. FFA conversions in brackets were obtained from GC/MS. **Residence time (min) is based on the volumetric liquid feed rate and reactor volume. ***SnO₂/SO₄ ² ⁻ denotes SnO₂ doped with sulphuric acid, ZrO₂/SO₄ ² ⁻ denotes sulphated zirconium oxyhydroxide, and TiO₂/Zr(SO₄)₂ denotes TiO₂ solid support doped with Zr(SO₄)₂.

Esterification produces a raw diesel stream 26 of approximately 50% alkyl esters (biodiesel) and 50% hydrocarbons. These hydrocarbons can include tetradecane, pentadecane, 1-hexadecene, hexadecane, heptadecane, 1-octadecene, octadecane, nonadecane, 1-eicosene, eicosane, heneicosane, 1-docosene, docosane, tricosane, tetracosane, pentacosane, hexacosane, heptacosane, octacosane, nonacosane, triacontane, untriacontane, dotriacontane, tritriacontane, tetratriacontane, pentatriacontane, hexatriacontane, heptatriacontane, and octatriacontane.

It should be noted that, in addition to esterifying only the middle distillates fraction 22 from thermal cracking, it is also possible to esterify both the naphtha stream 16 and middle distillates fraction 22 from the thermal cracking step. This optional method circumvents an extra step of separating naphtha 16 from the middle distillates 22.

Depending on the type of catalyst used and the degree of esterification achieved, the raw diesel stream 26 may exceed acidity limits allowed by ASTM specifications for biodiesel, namely 0.5 mg KOH/g. To reduce acidity, the raw diesel stream 26 can optionally be fed to a base treatment unit 30, together with a basic solution 28. The basic solution 28 reacts with any unreacted fatty acids in the raw diesel stream 26 to produce non-foaming metallic soaps with low solubility in bio-diesel. These non-foaming metallic soaps can then be separated by conventional known methods such as vacuum distillation, centrifugation, filtering or combinations thereof. The inventors note that the non-foaming metallic soaps, which include salts such as calcium, magnesium, potassium and lithium salts, have a high viscosity and can be sold as a valuable bio-lubricant by-product.

Base treatment is preferably carried out at temperatures of from 30 to 60° C., and more preferably at temperatures of from 40 to 50° C. and preferably at atmospheric pressure. The basic solution is preferably chosen from lithium hydroxide (LiOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂). Most preferred are LiOH and Ca(OH)₂.

The base treatment step results in a mixed biodiesel/diesel product 32 that has been found to have excellent fuel properties.

The boiling point distribution of the resultant biodiesel/diesel product 32 is found to be broader than that of biodiesel produced by conventional transesterification alone. The mixed biodiesel/diesel product 32 can be used both neat or can optionally be further blended with regular diesel.

The naphtha stream 16 from the thermal cracking unit 10 contains oxygenates and can optionally be sold as a valuable by-product such as octane improver. The residue stream 18 can be discarded by well known means in the art.

As mentioned earlier, the step of thermal cracking can optionally be replaced by a step of rapid pyrolysis. This process is shown in FIG. 2. Rapid pyrolysis is a process of decomposing a chemical at very high temperatures and in the absence of an oxidizing agent. Rapid pyrolysis has very short residence times when compared to thermal cracking.

In the present invention, rapid pyrolysis of triglycerides, and more particularly low quality or waste triglycerides, can be conducted at temperatures ranging from 480° C. to 600° C. for approximately 2 seconds. The triglycerides 12 are fed to a fluidized bed reactor 34 which is preferably fluidized with steam 36, although other suitable fluidizing media known in the art can also be used and are encompassed by the present invention. Steam 36 may be fed to the reactor at a ratio ranging from 0.5 to 1.5, relative to the triglyceride feed stream. The most preferred steam to triglyceride feed ratio is 0.9.

Any known inert gas 38 can optionally be added to the reactor to purge the reactor of free oxygen during pyrolysis. The inert gas 38 is preferably nitrogen. A catalyst may also be added, and suitable catalysts include, but are not limited to acid washed activated carbon, calcined sewage sludge solids and silica sand, such as silica alumina. The catalyst acts to enhance the selective cracking of triglyceride molecules to largely free fatty acid molecules.

Sample data of rapid pyrolysis conducted by the inventors on a trap grease feedstock is listed in Table 2 below. The resultant pyrolysis products are shown in Table 3.

TABLE 2 Rapid pyrolysis conditions Run ID 261 265 253 Temperature (° C.) 511 575 580 Fluidizing media Steam Steam Steam Steam/Feed ratio by ~0.9 ~0.9 ~0.9 weight N₂ purge/Feed ratio ~7 ~7 ~7 by weight Catalyst Acid washed Sewage sludge Silica sand activated carbon, solids, calcined 35 mesh minus at 750° C. Gas phase contact ~2 ~2 ~2 time (s)

TABLE 3 Rapid Pyrolysis Products 261 265 253 Gas 28.2 11.3 7.6 Liquid 50.3 89.4 90.7 Solids (coke) 9.0 Trace 1.4 Total above 87.5 100.7 99.7

The liquid fraction identified in Table 3 above contains middle distillates 22 as well as naphtha 16 and some residue 18. The boiling point distribution of the liquid fraction was determined by thermogravimetric analysis (TGA) and is given in Table 4 below. The middle distillates yield is given in Table 5. These tables indicate that rapid pyrolysis of triglycerides can produce an even larger proportion of desirable middle distillates than thermal cracking.

TABLE 4 Boiling point distribution of the liquid fraction (from TGA) 261 265 253 Naphtha (IBP ~165° C.) 86% 10%  8% Middle distillate 12% 75% 64% (165~345° C.) Residue (345° C. plus)  2% 15% 28%

TABLE 5 Middle distillate yield with respect to feed 261 265 253 Middle distillate 6% 67% 58% (wt % of feed)

The middle distillate fraction 22 produced by rapid pyrolysis was found to have varying free fatty acids (FFA) content, depending on the pyrolysis conditions. These details are shown in Table 6 below:

TABLE 6 Fatty acids in the middle distillate fraction Run ID 261 265A 265B 253 Pyrolysis Temperature 511 575 575 580 (° C.) Total FFA wt % 0.63 45.70 45.50 33.17

The present inventors noted that the largest middle distillates fraction was produced by rapid pyrolysis at a temperature of 575° C. As well, FFA content was highest for this temperature range. A preferred temperature range for rapid pyrolysis of the present process is therefore from 550° C. to 600° C. and a most preferred range is from 565° C. to 585° C.

The difference in middle distillates yield between the run at 575° C. and the run at 580° C. is thought to be due to the difference in catalysts rather than the small difference in temperature. Catalyst derived from sewage sludge is less acidic than silica sand. Thus, although the run with silica sand produced a slightly larger liquids fraction by deoxygenation, this was accompanied by higher coke and residue formation, resulting in an overall lower level of middle distillates. Thus the sewage sludge appears to provide a preferred balance between higher middle distillate yield and lower coke formation.

It has also been noted that the middle distillate stream produced by rapid pyrolysis comprises practically no nitrogen. Nitrogen content in the middle distillate obtained by mild thermal cracking was in the order of 5200 ppm whereas that in the middle distillate obtained by rapid pyrolysis was 0.3 ppm. This is particularly advantageous since the presence of nitrogen diminishes the quality of the final biodiesel product.

As well, total sulphur in the middle distillate obtained by mild thermal cracking was in the order of 500 ppm whereas that in the middle distillate obtained by rapid pyrolysis was 150 ppm. Both pre-treatment steps produce free fatty acids and other components containing sulphur and nitrogen. However, it is thought that products from rapid pyrolysis leave the reactor before the sulphur and nitrogen-containing components start to react with each other and become an integral part of the middle distillates fraction. Once nitrogen and sulphur enter the middle distillate stream, it can be very difficult to remove them from the final alkyl ester (biodiesel) product.

The following examples serve to better illustrate portions of the process of the present invention, without limiting the scope thereof:

Esterification Setup:

A metal screen was placed in the bottom of a stainless steel micro-reactor (reactor volume 10 mL) and covered with a thick layer of glass wool. The reactor was loaded with a measured amount of catalyst and was tightly shut. When the predetermined temperature was reached, vacuum was applied and two syringe pumps containing feedstock and methanol respectively were started and the feeds entered the microreactor. Vapour leaving the reactor was condensed and analyzed for FFA conversion.

Example 1

The esterification system consists of 1) a feed syringe pump, 2) a micro-reactor (10 mL), 3) a water-cooled condenser, 4) a room temperature trap, 5) an ice-water trap and 6) a mechanical vacuum pump. The vacuum pump attached to the exit side of the system maintained constant vacuum during the esterification. Thermally cracked palm oil and methanol were premixed at a weight ratio of 1:1 and loaded in an 8 mL syringe pump. Calcined TiO₂/Zr(SO₄)₂ in an amount of 4.1 g was charged in the micro-reactor between the layers of compacted glass wool and used as the acid catalyst. The catalyst occupied about 8 mL of the reactor volume. The reactor was heated to near 200° C., the vacuum pump was turned on and feed was started at a feed rate of 20 μL/min. The system pressure was maintained at 57 mmHg (1.1 psia) by bleeding a small stream of air into the vacuum pump inlet. Upon completion of the experiment after 7 h 45 mm, the total acid number (TAN) determination of the product was performed. The total amount of cracked palm oil was 10.84 g and the total amount of reacted oil (free of methanol) was 8.7 g. The TAN number of the feed mixture was 120.0 mg KOH/g and that of the product was 20.632 mg KOH/g. Thus, the free fatty acids conversion based on the TAN number was 82.8%.

Example 2

The esterification system consists of 1) a feed syringe pump, 2) a micro-reactor (10 mL), 3) a water-cooled condenser, 4) a room temperature trap, 5) a liquid-nitrogen trap, and 6) mechanical vacuum pump. The vacuum pump attached to the exit side of the system was intended to maintain the system pressure during esterification. Thermally cracked palm oil and methanol were premixed at a weight ratio of 1:1 and loaded in a 50 ml syringe pump. Calcined ZrO₂/SO₄ ²⁻ in an amount of 5.7 g was loaded into the micro-reactor as the solid acid catalyst, between the layers of compacted glass wool. The catalyst occupied 8.7 mL of the reactor volume. The reactor was heated to near 250° C., the vacuum pump was turned on and feed was started at a feed rate of 1000 μL/min. The system pressure was maintained at 5 mmHg (0.1 psia) by bleeding a small stream of air into the vacuum pump inlet. Upon completion of the experiment after 50 min, the samples were collected, mass balance performed, and the total acid number (TAN) was determined. The total amount of input (feed) was 40.0 g, the total amount of output was 36.0 g including 0.8 g collected on the catalyst bed. Thus, the mass balance for this experiment was 90.0%. The TAN number of the feed was 113 mg KOH/g and that of the product was 2.219 mg KOH/g. Thus, the free fatty acids conversion based on the TAN number was 98.0%.

This detailed description of the process and methods is used to illustrate certain embodiments of the present invention. It will be apparent to those skilled in the art that various modifications can be made in the present process and methods and that various alternative embodiments can be utilized. Therefore, it will be recognized that various modifications can also be made to the applications to which the method and processes are applied without departing from the scope of the invention, which is limited only by the appended claims. 

1. A method of producing biodiesel from a triglyceride feedstock, the method comprising: a. pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides to form a middle distillate fraction rich in free fatty acids; b. conducting vapour phase esterification of the middle distillate fraction under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a biodiesel stream; c. treating the biodiesel stream with a basic solution to convert residual free fatty acids to non-foaming metallic soaps; and d. separating the non-foaming metallic soaps by vacuum distillation, centrifugation, filtering or combinations thereof.
 2. The method of claim 1 wherein the basic solution is an aqueous solution of a compound selected from the group consisting of lithium hydroxide (LiOH), potassium hydroxide (KOH), magnesium hydroxide (Mg(OH)₂), and calcium hydroxide (Ca(OH)₂).
 3. The method of claim 1 wherein the triglyceride feedstock is selected from the group consisting of canola oil, palm oil, soy oil, corn oil, cottonseed oil, mustard seed oil, fish oils, algae oils, restaurant trap grease, rendered animal fats, waste greases, low-quality vegetable oils and combinations thereof.
 4. The method of claim 1 wherein thermal cracking is conducted at a temperature of from 390° C. to 460° C.
 5. The method of claim 1 wherein rapid pyrolysis is conducted at a temperature of from 480° C. to 600° C.
 6. The method of claim 5 wherein the triglyceride feedstock is fluidized with steam.
 7. The method of claim 6 wherein the steam to triglyceride feedstock ratio ranges from 0.5 to 1.5.
 8. The method of claim 5 wherein an inert gas is used to purge any oxidizing agents during rapid pyrolysis.
 9. The method of claim 5 wherein a catalyst is used during rapid pyrolysis to enhance the cracking of triglycerides to largely free fatty acids.
 10. The method of claim 9 wherein the catalyst is selected from the group consisting of acid washed activated carbon, calcined sewage sludge solids and silica sand.
 11. The method of claim 1 wherein esterification is conducted in the presence of methanol as the alcohol.
 12. The method of claim 1 wherein the ratio of middle distillate stream to alcohol is in a range of from 3:1 to 0.1:1.
 13. The method of claim 12 wherein the ratio of middle distillate stream to alcohol is in a range of from 2:1 to 1:1.
 14. The method of claim 1 wherein residence time for esterification ranges from 6 to 425 minutes.
 15. The method of claim 14 wherein residence time for esterification ranges 6 to 43 minutes.
 16. The method of claim 1 wherein esterification is conducted at a temperature of from 150 to 350° C.
 17. The method of claim 16 wherein esterification is conducted at a temperature of from 200 to 250° C.
 18. The method of claim 1 wherein esterification is conducted in a vacuum range of from 0.1 to 1.16 psia.
 19. The method of claim 1 wherein the solid acid catalyst is selected from the group consisting of a TiO₂ solid support doped with Zr(SO₄)₂, SnO₂ doped with sulphuric acid, sulphated zirconium oxide (ZrO₂/SO₄ ²⁻), sulphated iron oxide, halogenated alumina, sulphated tin oxide, trifluoromethyl-imines, tungstated zirconia-alumina (W/SiZr—Al) and silica-supported aluminum chloride.
 20. A method of producing a biodiesel/naphtha mixture from a triglyceride feedstock, the method comprising: a. pretreating the triglyceride feedstock by thermal cracking or rapid pyrolysis to remove contaminants and convert triglycerides to produce a middle distillate fraction rich in free fatty acids, a naphtha stream and a gas stream; b. conducting vapour phase esterification of the middle distillate fraction under vacuum and in the presence of an alcohol and a solid acid catalyst to produce a mixed biodiesel/naphtha stream; c. treating the mixed biodiesel/naphtha stream with a basic solution to convert residual free fatty acids to non-foaming metallic soaps; and d. separating the non-foaming metallic soaps by vacuum distillation, centrifugation, filtering or combinations thereof. 